The present invention relates to a machine for wrapping elements of boxlike and substantially parallelepiped embodiment.
In particular, the present invention relates to a machine by which substantially paralleleliped box elements are enveloped in wrappers of heat-sealable material.
The prior art embraces numerous wrapping machines, and more especially cellophane wrapping machines, by which substantially parallelepiped commodities, notably packs of cigarettes, can be enveloped both individually and in groups. Such machines generally comprise means by which to feed and subsequently index the boxes or packs, also folding means, and sealing means.
Each box element is urged by the feed means against a respective wrapper, which includes a sheet of the heat-sealable material supplied in a direction perpendicular to the feed direction, and directed ultimately into a recess together with the wrapper.
The dimensions of the recess are substantially identical to those of the box element, such that upon insertion, the wrapper is forced into a `U` shape around the element by the side walls of the recess, with two extremities or flaps projecting.
The folding means, which include a moving folder and a fixed folder, serve to fashion each wrapper into a tubular sheath around the respective box element by engaging the two projecting flaps and flattening them into overlapping contact against one side or face of the element.
Finally, the two overlapping flaps are secured one to the other by the sealing means and the tubular configuration is thus rendered stable.
In many machines, the recess include one of a plurality of radial pockets provided by a rotary wrapping head that is indexed about a horizontal axis through a number of work stations.
The moving folder effects a reciprocating movement, and serves to flatten the lower of the projecting flaps against the rearwardmost face of the box element at a moment immediately following the entry of the element and the corresponding wrapper into the pocket, and immediately prior to the indexing movement of the head.
The fixed folder is provided by the bottom edge of an external hood, or casing, disposed coaxial with the wrapping head in such a way that subsequent rotation of the head has the effect of flattening the upper projecting flap of material over the already folded lower flap and thus completing the operation whereby the wrapper is fashioned into a tubular sheath around the box element.
While it is true that high operating speeds are obtainable with a structure of the type outlined above, there are also certain drawbacks as regards a correct and durable fold of the wrappers around the corresponding box elements.
A correctly folded wrapper is of great importance nonetheless, and a determining factor contributing to the ultimate appearance of the end product.
Accordingly, it is essential that the wrapper be properly taut when folded around the respective box element.
The drawbacks in question derive first and foremost from the fact that, even with the projecting flaps of the wrapper properly folded against the relative side or face of the box element, the requisite tubular configuration of the wrapper does not hold sufficiently stable through until the subsequent heat-sealing operation which, as already suggested, occurs at a time and location posterior to those of the folding operation.
This problem is aggravated in the case of the expedient described above, whereby the box element and the ensheathing wrapper are held in place in the respective pocket, during the rotation of the wrapping head, through the agency of an essentially cylindrical hood or casing. In the course of such rotation, in effect, contact with the inner surface of the casing is limited to the longitudinal corner edges of the exposed face of the box element.
Besides being unable to guarantee that the correct tubular configuration of the wrapper is maintained, the contact in question is difficult to achieve and maintain for two contrasting reasons, namely: too hard a contact may well result in a damaging compression of the longitudinal edges of the rear face of the box element, as well as causing the lower flap, pinched between the fixed casing and the indexing element, to slide away from beneath the upper flap; by contrast, insufficient pressure in the contact between the casing and the element will result in the two flaps being allowed an undue degree of freedom, such that the wrapping material can slacken and the requisite tautness be lost.
Moreover, the wrapper is embodied in an extremely thin and pliable material that readily accumulates an electrostatic charge, a characteristic which complicates the operation of the moving folder, caused as it is to slide in close contact with the lower of the two projecting flaps to the end of flattening the material tightly against the box element. This same sliding action has the effect of inducing an electrostatic charge in the flap, which consequently tends to cling and is drawn back by the folder on the return stroke.
The problem is compounded by the fact that these electrostatic charges increase proportionally with any increase in the velocity of the sliding contact between folder and flap, that is to say with higher operating speeds of the wrapping machine.
As may readily be deduced, in effect, the moving folder tends to drag the lower flap such that it bunches beneath the upper flap, with the inevitable result that the requisite olean, flush overlapping contact cannot be obtained subsequently.
This bunching or creasing of the lower flap leads in turn to a further drawback during the subsequent heat-sealing operation, namely: the fact that one of the two overlapping extremities of the wrapper may be creased signifies a reduction in the surface areas effectively in contact, hence an inevitable reduction in dependability of the heat-sealed joint.